1. Field of the Invention
The present embodiment relates to rare earth cold accumulating material particles, and a refrigerator, a superconducting magnet, an inspection device and a cryopump using the same.
2. Description of Related Art
Recently, the development of the superconducting technology has been remarkable, and with the expansion of the application fields of the superconducting technology, the development of small-size and high-performance refrigerators has been essential. Examples of the type of the refrigerator include various types such as a GM (Gifford-McMahon) type, a pulse type and a Stirling type. Examples of the products using these refrigerators include a superconducting magnet, MRI, NMR, a cryopump, a superconducting magnetic energy storage (SMES) and a single crystal pulling-up apparatus in a magnetic field for producing silicon wafers and the like.
In such a refrigerator, a working medium such as compressed He gas flows in one direction in a cold accumulator filled with a cold accumulating material, the thermal energy of the working medium is supplied to the cold accumulating material, and the expanded working medium flows in the opposite direction and receives the thermal energy from the cold accumulating material. As the recuperative effect in such a process becomes satisfactory, the thermal efficiency in the working medium cycle is improved, and it is made possible to actualize lower temperatures.
As such a cold accumulating material as described above filled in the cold accumulator of a refrigerator, Cu, Pb and the like have hitherto been mainly used. However, such a cold accumulating material has a remarkably small specific heat at ultralow temperatures of 20K or lower, accordingly does not allow the above-described recuperative effect to sufficiently function, cannot store a sufficient thermal energy in the cold accumulating material every one cycle at an ultralow temperature during the action in a refrigerator, does not allow the working medium to receive a sufficient thermal energy from the cold accumulating material.
Consequently, a refrigerator incorporating the cold accumulator filled with the cold accumulating material suffers from a problem that such a refrigerator is not allowed to reach an ultralow temperature. Thus, nowadays, in order to improve the recuperation property at ultralow temperatures of the cold accumulator and to actualize a refrigeration temperature closer to the absolute zero temperature, in particular, there are used rare earth cold accumulating materials mainly composed of intermetallic compounds including rare earth elements and transition metal elements such as Er3Ni, ErNi and HoCu2 each having a local maximum value of the volume specific heat in the ultralow temperature region of 20K or lower wherein the local maximum value is large. By using such a rare earth cold accumulating material in a GM refrigerator, the refrigeration at 4K is actualized.
Along with the developed investigation of the application of such a refrigerator to various systems, from the technical requirements for stably cooling larger-scale cooling objects, refrigerators are required to be further improved in refrigerating capacity. In order to meet the requirements, recently, an attempt has been made to improve the refrigerating capacity by replacing part of a conventional metal-based magnetic cold accumulating material with a rare earth element-containing rare earth oxysulfide such as Gd2O2S.
A rare earth oxysulfide has a peak of the specific heat at 5K or lower, which is lower than the peak temperature of the specific heat of a rare earth cold accumulating material. Accordingly, an improvement of the refrigerating capacity can be achieved by using a rare earth oxysulfide as laminated on a rare earth cold accumulating material mainly composed of an intermetallic compound, having a large volume specific heat in the temperature region of 6K or higher. A rare earth oxide cold accumulating material such as GdAlO3 has a low specific heat peak, and provides an effect similar to the effect obtained from a rare earth oxysulfide cold accumulating material.
Rare earth oxysulfide cold accumulating materials are disclosed in Japanese Patent No. 3642486 (Patent Document 1) and Japanese Patent No. 4582994 (Patent Document 2). In Patent Document 1 and Patent Document 2, high density sintered bodies each having a relative density of 98% or more are obtained by using a tumbling granulation method.